Engine Trouble Running Essentials- a complete troubleshooting guide

Discussion in '2-Stroke Engines' started by jaguar, Nov 3, 2015.

  1. jaguar

    jaguar Well-Known Member

    Running Essentials:
    1) gasoline/air mixture (mixed not too lean or too rich)
    2) compression (both in crankcase and combustion chamber)
    3) spark (of sufficient strength and at the right time each cycle)

    Incorrect fuel mixture ratio:
    A) Vacuum in the gas tank due to lack of functioning vent hole will let the engine run for a while and then die. Taking the cap off to see if there is fuel there relieves the vacuum. The spark plug will not be wet when removed and inspected. Drill a small hole in the bottom lip of the cap.
    B) Trash all around the fuel filter in the tank can prevent fuel flow. Remove the petcock to inspect and clean it.
    C) Trash inside the float bowl can get into the main jet and prevent fuel transfer into the throat of the carb. Use bare wire from a twist tie to push into the main jet to break free any obstructions.
    D) A carb float valve not sealing will allow the fuel to fill up too high in the float bowl to make it run rich.
    Testing: Take the float bowl off and turn the gas petcock on and move the float up and down to see if the fuel flow stops when the float is up. If bad then clean the valve and if it still continues then replace the carb. If good then adjust the float tab so that the on/off point is 21mm from the carb body (see http://www.dragonfly75.com/motorbike/jetting.html).
    E) A restrictive air filter will cause it to run too rich. Remove the air filter and seat it on your face around your mouth and blow hard. If you can feel its resistance to air flow then it is too restrictive. I made my own by gluing (with silicone sealant) together foam sections cut from a lawn mower filter.
    F) An air leak at the beginning or end of the intake manifold can cause it to run too lean. With the clutch lever held in does it take a while for the idle to settle down to its normal putt-putt after twisting the throttle and then letting it go? That would indicate a leak at the seals or at the intake manifold (usually where the carb connects to it). Use gasoline resistant glue (not silicone sealant) at the intake manifold if necessary.


    Clutch Slipping
    It's possible the clutch is slipping too much to let the engine turn over. With the left cover off just look to see if the magneto is rotating when you push the bike. If not then adjust the clutch if there is some slack in the clutch cable (which is necessary)

    Air Leak at the crank seals:
    A leak here will lose crankcase compression which reduces power, sometimes to the point of not even being able to start the engine.
    The right side of the crankshaft has a lip that the seal can catch on, so even a new engine can have that seal leaking due to not seating correctly. I like to use a file to round that lip off.
    take the magneto and stator coil off.
    take the spark plug off.
    spray the seal area with soapy water (using dishwashing soap).
    turn the engine over and watch for bubbles.
    then on the right side take the primary gear off and do the same.
    bubbles indicate a seal leak.

    Air leak at the head/cylinder:
    Most of these heads come slightly warped from the factory and the aluminum gasket is meant to be thrown away the first time it is removed (because it can only conform to irregularities once). A loss of compression here greatly reduces engine power or the ability to start. Plane the head flat by use of sandpaper on thick glass. Buy gasket material from the auto parts store and make your own gasket. With patience and effort you can also keep grinding down the head to remove up to 2mm so that the compression will increase to around 135psi (higher is not recommended due to weak cylinder plating). Stock is around 90psi if the head is sealing good. But don't take off much more metal than what is needed to make it flat unless you buy a Jaguar CDI which has the retarded ignition timing needed for higher compression. (see http://www.dragonfly75.com/motorbike/Compress.html)

    Unless you are checking for spark with a timing light while it's running (because the light won't turn on unless there is a surge of current going thru the wire to the spark plug which indicates a spark event) you cannot be sure the spark is happening. It can spark outside the engine with the plug removed but not inside the engine. Why? The voltage needed to make a spark increases with pressure increase.
    The current return path can be intermittent if you haven't removed the shellac from where the 4 screws contact the stator coil frame. (see https://www.youtube.com/watch?v=Nd2G9FB1nJk)
    The stator coil can have an intermittent short (voltage dependent).
    The CDI can be intermittently faulty.
    The magnet can be installed backwards which will make the ignition timing so far off that the engine won't start.
    The spark plug may be damaged or too dirty and needing replacement.
    Make sure the plug gap does not exceed .025" because more gap requires more voltage to create a spark.
    If you have a kill switch then disconnect the wire going to it to see if it is the problem.
    Make sure the wires going from the stator coil to the CDI have good connections. Two rusty wires twisted together does not make for good conduction.

    Testing the spark ignition:
    Remove the spark plug and connect the spark plug cap from the high voltage coil to it. Manually hold the spark plug with cloth onto the cylinder head and make sure the side electrode is sideways so you can see the spark gap clearly. Push the bike to see if any spark is generated. It is hard to see in full daylight so its best to do at night or inside. If no spark happens then there is a problem. If you have spark but the engine won't start then one of these problems exist:
    1. Compression is being lost due to loose head nuts or non-parallel head/cylinder mating surfaces (that you can fix with sandpaper and glass).
    2. There is an input air leak due to loose intake manifold connections or poor sealing where the carburetor connects to the intake manifold.
    3. Crankcase compression is being lost due to bad crankshaft seals.
    4. Something is really wrong with the carburetor such as a jet being clogged.
    5. The ignition system is just strong enough to create a spark with the spark plug outside the engine but not inside the engine.
    6. Someone put the magneto on backwards so that the spark is happening at the wrong time. This photo shows the magneto correctly installed with the piston at top dead center:
    Screen Shot 2015-11-03 at 2.41.28 PM.png

    Test the Stator Coil:
    Use an AC voltmeter to test for 25-50 volts coming out of the stator coil as you push the bike with the spark plug removed. I prefer using an analog voltmeter such as this one: http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2130765_-1
    If the voltage is there then the stator coil is OK although I would still remove the stator coils mounting screw that fastens the ground wire to the metal ground so that you can clean it for good conduction. If the voltage is too low then either the stator coil has a short circuit in it or the magnet is weak and needs to be replaced. Usually the coil is the problem.

    Test the CDI:
    You can test for total circuit performance (without determining the peak output voltage which requires a special test circuit or a meter that can read peak voltage) by just holding the bare CDI output wires while pushing the bike. Someone may have to help you with that. Start out very slow and gradually push faster till you feel a minor shock. I prefer this basic method because the output voltage lasts for only a few thousandths of a second which most meters can't register. If you are getting shocked then suspect the high voltage coil or the spark plug cap or the spark plug as bad.
    Last edited by a moderator: Dec 15, 2015

  2. jaguar

    jaguar Well-Known Member

  3. jaguar

    jaguar Well-Known Member

    If you test the spark plug outside of the combustion chamber then use an old plug gapped to .07" (1.8mm) which requires around 7000 volts for a spark (a typical running voltage for spark). That will insure your ignition system is making enough voltage to make a spark inside the combustion chamber where the high pressure makes it harder for a spark to occur. With a plug gap of .025" (.65mm) and the plug outside the combustion chamber it only takes around 3500 volts to make a spark.
    FurryOnTheInside likes this.
  4. mrbg

    mrbg Member

    Thanks, that's what I'm checking next spark. Suspicious that the previous cdi worked the same day with the old motor. New balanced motor has a high compression stock head so maybe the old cdi has a weaker spark. The motor is still really tight I have to pedal a bit longer before I dump the clutch and the tire skids a bit before the motor fires but I don't see the connection to the topend bog.
  5. jaguar

    jaguar Well-Known Member

    tire skidding means your compression is too high. you'll quickly wear out the needle roller bearing at the top of the connecting rod. Also it causes too high a piston and head temperature. but it doesn't cause bogging at high RPM.

    Bogging- if you don't have an intake air leak or a leak at the crank seals then you have to change the hole size in your main jet. My site tells how to do that. (click on my signature link)
  6. crassius

    crassius Well-Known Member

    easing out the clutch instead of dumping it will save wear on clutch, engine bearings, and tires
    Jeffrey Hartman likes this.
  7. mrbg

    mrbg Member

    That's how the motor came, in the specs it's listed high compression head. I spoke to the vendor and he told to check for airleak with a carb cleaner!! And for the tire skidding its been in the morning when it's cold. I'm aware of the clutch pads being abused also concerned of ripping my valve stem. What am I to do it's basically a brand new motor 2 tanks of gas. I told him about my old cdi not working with this motor, he couldn't explain that. This shop is off of butterfly 2 stroke mods, I checked your link download, great stuff. I'm still kinda miffed by the way it was handled, I won't be able to work on it for a couple of days.
    Last edited: Nov 30, 2015
  8. mrbg

    mrbg Member

    Hey one more thing my other bike it runs good but recently when I come to a stop from cruising speed 25 mph it idles high for a bit. I've had that happen on another bike when was kinda Worn out. But overall performance is good so I didn't mess with it. Another airleak maybe ?
  9. crassius

    crassius Well-Known Member

    air leak or sticky slide both do that - list to hear click as slide hits bottom
  10. mrbg

    mrbg Member


    not butterfly it's dragonfly that's what meant to say
  11. mrbg

    mrbg Member

    Tomorrow I'm going to work on the bike, I hope it's an fix. One other thing I noticed Is the head bolts are smaller and they haven't budged
  12. bcredneck

    bcredneck New Member

    two things I noticed you said but should never do never push wire into your jet they are soft brass and the shape of the venturi can be easily damaged even copper wire and small pressure will affect the edges of the internal taper

    and never use silicone anywhere gas go's yes they use it for head gaskets on car engines but it doesn't touch much fuel when applied properly on your air filter it going to let loose and go inside the motor it won't cause much damage going through but most of the time a chunk will hold your slide up and your stuck at half or more throttle

    that almost killed a friend of mine on a Dr 370 after he jumped off the bike it hit a tree at around 70 and wrapped his front rim around the forks he was badly hurt just jumping on grass at that speed
    so use seal all its gas /oil safe
  13. AquaManAndy

    AquaManAndy Member

    If your engine compression is too high how can you lower it?
  14. Street Ryderz

    Street Ryderz Well-Known Member

    Raise the head with a thicker gasket or two,Raise the whole cylinder two base gaskets works well but slightly alters timing!
  15. jaguar

    jaguar Well-Known Member

    raising the cylinder benefits low rpm power at the expense of high rpm power.
  16. Jeffrey Hartman

    Jeffrey Hartman New Member

    I have a question. Just put new piston,rings,gaskets. But now piston stroke rises too high locking up cause cannot complete it's revolution upto combustion chamber?? What could cause this? Everything the same.?
  17. 6blueoval9

    6blueoval9 Active Member

    Possibly the wrong piston...there are two types...a low pin and a high pin...a high pin piston would possibly be too tall for a short pin type motor and hit the combustion chamber of the head and lock up from completing a revolution.
  18. Jeffrey Hartman

    Jeffrey Hartman New Member

    Thought they were all standard..the same.. Will check that out.. Thanks for the feed back..much appreciated!